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National Marine Mammal Laboratory (NMML)

Alaska Ecosystems Program

Life History and Distribution Patterns of Fishes and Cephalopods: A Window into the Foraging Ecology of Pinnipeds

Figure 1. Three species of squid from the family Gonatidae: Gonatopsis borealis, Berryteuthis magister, and Gonatus sp. caught in a single haul in the Bering Sea greenbelt. These specimens range from 18 to 60 cm in length and are examples of prey items critical to marine mammal diets throughout the Bering Sea.

Prey studies are critical to understanding the foraging behavior of threatened and endangered pinnipeds. They are instrumental in our ability to differentiate (and possibly mitigate) the relative impacts of climate vs anthropogenic (commercial fisheries) driven causes of population decline. The Alaska Ecosystems program (AEP) at the National Marine Mammal Laboratory (NMML) researches the ecology of common prey species in conjunction with annual monitoring of pinniped diets. In the eastern Bering Sea and Gulf of Alaska, these include fishes of commercial (i.e., walleye pollock, Theragra chalcogramma) and non-commercial (i.e., deepsea smelt, Leuroglossus schmidti) importance and squid of the family Gonatidae (Figs. 1 and 2).

Figure 2. This snailfish (probably Elassodiscus tremenbundus) collected in the southeastern Bering Sea is common in Steller sea lion (Eumetopias jubatus) diets in specific locations along the Aleutian Islands.

An example of NMML participation in cooperative research investigating broad aspects of the North Pacific ecosystem includes a mesopelagic trawl survey conducted in May 1999 and 2000. As a partner with Fisheries-Oceanography Coordinated Investigations (FOCI) and with funding from the Arctic Research Institute, AEP scientists conducted research trawl operations to depths of 1,200 m specifically targeting forage fishes and squid which are not otherwise monitored (Sinclair and Stabeno 2002, Deep Sea Research II).

A unique trawl net was designed and constructed for these studies. Directed fishing effort in concert with oceanographic measurements provided multi-layered profiles of the distribution, life history, and energetic value of common marine mammal prey species. The contribution of these collections to our understanding of pinniped foraging ecology continues to unfold as new techniques in animal tracking and diet analyses develop (Kurle et al. 2011, Marine Biology).

One key benefit of directed forage fish surveys is the collection and proper identification of a size-­stratified series of prey bone samples (Fig. 3). For 60 years, pinniped gut and scat samples have been analyzed for the bony remains of their prey without exact identification of many species common in their diet. The directed forage fish surveys expanded the NMML reference collection of forage fish and cephalopod remains, as they appear in juvenile and adult life stages.

Size-stratified bony remains, together with collection location of prey, indicate where and at what level of the water column predators are foraging. Identification of prey species through scat and gut analyses is now dramatically improved. A secondary application of prey collections is quantifying the bio-energetic value of a single meal. Ultimately, meal values are applicable to ecosystem-wide modeling efforts, since many of these prey species are also common in the diets of seabirds and groundfish.

Figure 3. An illustration of beaks from Berryteuthis magister, a dominant species in the diet of marine mammals throughout the Gulf of Alaska and Bering Sea. A size-stratified series of this squid species was collected in the eastern Bering Sea and contributed to the NMML reference collection and library of prey energetic values.

Defining the influence of climate on prey dynamics has become increasingly applicable to interpreting trends in diet and population of northern fur seals (Callorhinus ursinus) on the Pribilof Islands, home to the largest breeding colonies of fur seals and seabirds in the North Pacific. Recent analyses demonstrate a close link between sea-surface temperatures, the timing of ice retreat, and shifts in predator diets over the past 35 years (Sinclair et al. 2008, Deep Sea Research II). Ocean conditions directly influence the movements and year-class success of forage fishes and cephalopods. Shifts in the distribution and abundance of prey are reflected in seal diets and potentially in predator population trends over time.

Our understanding of the mercurial linkages between climate, diet, and population trends of marine predators is rudimentary. However, a research emphasis on the life history and distribution of their prey has furthered our understanding of factors influencing foraging behavior and trends in predator populations that continue to expand through the years.